1. Field of the Invention
This invention relates to a method and apparatus for transmitting digital video signals obtained on imaging by a CCD imaging device.
2. Description of the Related Art
Up to now, there has been known a system in which, as a system for digitally transmitting digital color video signals obtained on imaging by a CCD imaging device, a portable television camera (camera head), and a camera control unit provided in a relaying car, editing room or an imaging room, are interconnected by a cable, such as a coaxial cable for digitally transmitting color image signals imaged by the camera head to the camera control unit.
In such digital transmission system for transmitting digital color video signals between the camera head and the camera control unit, the sampling rate is set so as to be the same as that for digital imaging signals outputted by the camera control unit, or as the sampling frequency (sampling rate) of a charge coupled device (CCD).
FIG. 1 shows an example for transmitting digital color imaging signals from the camera head to the camera control unit. The arrangement shown in FIG. 1 uses a so-called multiple CCD type CCD camera head for a camera head in which the CCDs are offset relative to one another for suppressing pseudo signals and improving the resolution. This technique is known as a pixel offsetting technique. While the imaging signals of the frequency not less than one-half the sampling frequency represent pseudo signals in the single CCD type CCD camera head, even the imaging signals of a frequency not less than one-half the sampling frequency cease to be pseudo signals in the multiple CCD type CCD camera head employing the pixel offsetting technique, thus realizing high resolution.
Referring to FIG. 1, the light from an object, incident on a lens system 201 of a camera head 200, is separated by a color separation prism 202 into light beams of three colors, namely red (R), green (G) and blue (B). The light beams of R, G and B are incident on solid-state imaging devices (CCDs) 203a, 203b and 203c for conversion into electrical signals corresponding to the R, G and B colors, respectively. For exploiting the technique of the pixel offsetting technique, the CCDs 203R, 203G and 203B are arranged so that the CCD 203R and the CCD 203B are horizontally offset relative to the CCD 203G by one-half the pixel pitch. Consequently, the image incident on the CCDs 203R, 203G and 203B has imaging signals for G complementally sampled with respect to the imaging signals for R and B.
The imaging signals for R, G and B from the CCDs 203R, 203G and 203B are amplified by associated pre-amplifiers 204R, 204G and 204B, respectively, so as to be then converted by analog/digital (A/D) converters 205R, 205G and 205B, respectively, into digital imaging signals. It is noted that the A/D converters 205R, 205G and 205B convert analog imaging signals into digital signals using clocks of the same frequency as the sampling clocks used in the CCDs 203R, 203G and 203B, while the A/D converter 205G for G perform digital conversion with clocks delayed in phase by 180.degree. from those used in the A/D converters 205R and 205B for R and B, respectively, for realizing the pixel offsetting effect. In the example of FIG. 1, the frequency of the digital imaging signals outputted by the (A/D) converters 205R, 205G and 205B is 18 MHz.
The digital imaging signals outputted by the (A/D) converters 205R, 205G and 205B are sent to a digital processing circuit 206 which then performs so-called contour enhancement or gamma (.gamma.) processing on the digital imaging signals supplied thereto. The present digital processing circuit 206 performs processing with the frequency twice the frequency of the CCD sampling clocks for maintaining the pixel offsetting effect. Thus the frequency of the digital imaging signals obtained on processing by the digital processing circuit 206 is twice the frequency of the input digital imaging signals (18 MHz), or 36 MHz.
If a format of so-called SMPTE (Society of Motion Picture and Television Engineers) 295M composite signals, for example, is used as a format for digital video signals handled between the camera head 200 and a control unit 210, the frequency of the digital imaging signals obtained by the digital processing circuit 206 (36 MHz) is converted into a rate of, for example, 13.5 MHz. To this end, the digital imaging signals obtained by the digital processing circuit 206 are sent to a frequency conversion circuit 207 for converting the signal with the rate of 36 MHz into a signals with a rate of 13.5 MHz. In the above example, the rate of 13.5 MHz is used, however, if the rate of 18 MHz which is the same as the sampling frequency of the imaging signal output of the CCD is used, the frequency of the digital imaging signals from the digital processing circuit 206 (36 MHz) is converted by the frequency conversion circuit 207 into the frequency of 18 MHz. Meanwhile, the SMPTE is the digital standard for composite signals, and the SMPTE 295M provides the digital encoding standard for the NTSC signals and the bit-serial interfacing standard. The SMPTE provides for two sorts of the sampling frequency, that is 13.5 MHz and 18 MHz, according to Recommendations 601. The latter frequency of 18 MHz is a frequency raised from 13.5 MHz in proportion to the increased aspect ratio (13.5 4/3).
The R, G and B component signals, rate-converted by the frequency conversion circuit 207, are converted by a parallel/serial conversion circuit (P/S conversion circuit) 208 into composite signals of the above-mentioned SMPTE 295M standard so as to be outputted along with various other sorts of the information at an output terminal 209.
The composite digital imaging signals, outputted at the output terminal 209, are supplied to an input terminal 211 of the camera control unit 210 via a cable along with the other information.
The composite digital imaging signals, supplied to the camera control unit 210, are converted by a serial/parallel conversion circuit (S/P conversion circuit) 212 into component digital imaging signals of R, G and B. These component digital imaging signals of R, G and B are converted into analog imaging signals by digital/analog (D/A) converting circuits 213R, 213G and 213B associated with R, G and B, respectively, so as to be outputted at output terminals 214R, 214G and 214B, similarly associated with R, G and B, respectively.
If the composite digital video signals of the above-mentioned SMPTE 295M standard are outputted by the camera control unit 210, the R, G and B component digital video signals from the S/P conversion circuit 212 are converted by the P/S conversion circuit 216 into composite signals of the above-mentioned SMPTE 295M standard which are outputted at an output terminal 217. Meanwhile, if the rate of the component digital video signals of R, G and B from the S/P conversion circuit 212 S, for example, 18 MHz, it is converted by a frequency converter 215 into the rate of 13.5 MHz and thence supplied to the P/S conversion circuit 216.
However, in the above-described system, there is raised no particular problem if the camera control unit 210 outputs only the above-mentioned digital video signals. However, if the camera control unit 210 outputs an analog video signal, the effect of pixel offsetting by the camera head 200 is annulled as concerns the resolution. Stated differently, if the sampling frequency of transmission from the camera head 200 to the camera control unit 210 is the above-mentioned frequency, the pixel offsetting in raising the resolution of the analog video signals obtained by the camera control unit 210 is not effective for the analog video signals obtained from the camera control unit 210. More specifically, the limit resolution obtained from the 36 MHz rate digital video signals obtained with the digital processing circuit 206 of the camera head 200 is ideally approximately 1400. However, only video signals with the resolution of approximately 540 or approximately 720 can be obtained from the digital video signals of, for example, the rate of 13.5 MHz or the rate of 18 MHz , obtained after transmission from the camera head 200 to the camera control unit 210, as in the system shown in FIG. 1, respectively, thus lowering the resolution.